Why is today the first day of Summer?

Zugarramurdi, Spain

The first day of summer—heralded today by a manic bunny and bear in a Google doodle by artist Takashi Murakami—officially kicks off today at 1:16 p.m. ET (in the USA), the beginning of the summer solstice and of the longest day of the year, at least in the Northern Hemisphere. The summer solstice is a result of the Earth's north-south axis being tilted 23.4 degrees relative to the sun. The tilt causes different amounts of sunlight to reach different regions of the planet. Today the North Pole is tipped more toward the sun than on any other day of 2011. The opposite holds true for the Southern Hemisphere, where today is the winter solstice, the shortest day of the year. As a result, at high noon on the first day of summer, the sun appears at its highest point in the sky—its most directly overhead position—in the Northern Hemisphere. (Read a whole lot more on National Geographic.)